Problem 129 - Objective Biology for NEET

Question

Assertion: Rhizobium lives as aerobes under free living condition. Reason: Nitrogenase is operational under aerobic condition.

Step-by-Step Solution

Answer

The assertion is correct, but the reason given is incorrect.

Step 1: Understanding the Biological Concept

The assertion states that Rhizobium lives as aerobes under free living conditions, which means that it requires oxygen to live when it's not in symbiosis with leguminous plants. The reason given is that nitrogenase is operational under aerobic condition.

Step 2: Checking the Nitrogenase Function

Nitrogenase is an enzyme that converts atmospheric nitrogen \(N_2\) into ammonia \((NH_3)\), which is usable by plants. However, it is noted that nitrogenase is inhibited by oxygen. It is more operational under anaerobic conditions, not aerobic conditions.

Step 3: Conclude

The reason given is incorrect. Though Rhizobium is aerobic under free living conditions, nitrogenase does not operate under aerobic conditions. Instead, it operates mostly under anaerobic conditions. Therefore, the reason does not correctly explain the assertion.

Key Concepts

Aerobic and Anaerobic Conditions

In the world of microbiology, understanding the difference between aerobic and anaerobic conditions is vital. Aerobic means the presence of oxygen. Many organisms, like Rhizobium, require oxygen to survive when living freely outside host plants. In aerobic conditions, oxygen is used to produce energy efficiently.
However, some processes, such as the activity of the nitrogenase enzyme, occur more effectively in anaerobic conditions, meaning in the absence of oxygen. This is because oxygen can inhibit or damage these processes. In the case of nitrogenase, that means its functionality is impeded when oxygen is present.

• Aerobic = Presence of oxygen


• Anaerobic = Absence of oxygen

For Rhizobium, managing oxygen levels is crucial when it performs nitrogen fixation. Even though it can live aerobically, it creates microenvironments that limit oxygen exposure to keep the nitrogenase enzyme functional.

Nitrogenase Enzyme

The nitrogenase enzyme plays a pivotal role in the nitrogen cycle by converting atmospheric nitrogen ( \(N_2\) ) into ammonia ( \(NH_3\) ), a form of nitrogen that plants can readily use to synthesize proteins and other essential compounds. This process is called nitrogen fixation, and it occurs naturally in specific bacteria, including Rhizobium.
Nitrogenase is highly sensitive to oxygen, which can denature the enzyme and stop it from working. Therefore, bacteria that employ nitrogenase have developed unique methods to protect this vital enzyme from exposure to oxygen.
Rhizobium, for instance, may use leghemoglobin, a molecule that tightly binds to oxygen, creating a low-oxygen environment essential for nitrogenase activity. This way, Rhizobium can still utilize oxygen for its survival while keeping nitrogenase operational.

Symbiosis with Leguminous Plants

Symbiosis is a mutually beneficial relationship between two different organisms. Rhizobium has a unique symbiotic relationship with leguminous plants, such as peas, beans, and lentils. These plants form nodules on their roots, providing a safe haven for the bacteria.
Inside these root nodules, Rhizobium shifts from living aerobically to functioning in an environment where nitrogenase can work effectively. In return for the ammonia produced by nitrogen fixation, the plant supplies Rhizobium with carbohydrates that serve as a food source. It is a beautiful example of cooperation, where both the plant and the bacteria benefit from each other.

• Legumes provide shelter and nutrients


• Rhizobium offers fixed nitrogen, crucial for plant growth

This symbiotic relationship is crucial for agriculture, enhancing soil fertility by enriching it with nitrogen, reducing the need for synthetic fertilizers.